With lattes you can fit a double handle over the cup that you're making the coffee in. But a demitasse won't fit under a double handle. Do you make two single shots and pour the second one into the first? Do you fill a single handle twice?
It seems OK to prepare two single shots of espresso, then fill both of them in the same cup. It shouldn't be a serious problem except disturbing the foam form during filling.
I assume that contact with air, temperature difference, etc. could make subtle differences, but it will be still drinkable.